The present invention relates to a phase error measuring method and apparatus, phase error correcting method and apparatus, recording medium and magnetic resonance imaging apparatus, and more particularly to a method and apparatus for measuring a phase error in spins due to the effect of residual magnetization etc., a method and apparatus for correcting such a phase error, a recording medium recorded with a program for operating a computer to perform a phase error measuring function, a recording medium recorded with a program for operating a computer to perform a phase error correcting function, a magnetic resonance imaging apparatus comprising phase error measuring means, and a magnetic resonance imaging apparatus comprising phase error correcting means.
In a magnetic resonance imaging (MRI) apparatus, an object to be imaged is carried into an internal space of a magnet system, i.e., a space in which a static magnetic field is generated; gradient magnetic fields and a high frequency magnetic field are applied to generate magnetic resonance signals from spins within the object; and a tomographic image is reconstructed based on the received signals.
The gradient magnetic fields are applied in three mutually perpendicular axis directions. The three mutually perpendicular axes are slice, phase and frequency axes. The gradient magnetic field in the slice axis direction serves to selectively excite a desired slice on the slice axis by an RF (radio frequency) excitation signal, and is sometimes referred to as a slice gradient. The gradient magnetic field in the phase axis direction serves for phase encoding of the spins, and is sometimes referred to as a phase encoding gradient. The gradient magnetic field in the frequency axis direction serves for readout of the magnetic resonance signal, and is sometimes referred to as a readout gradient. The magnetic resonance signal is read out as an echo signal.
One magnetic resonance imaging method is the fast spin echo (FSE) technique. This technique involves exciting spins by 90xc2x0, followed by repeating inversion of the spins by a 180xc2x0 excitation a plurality of times to acquire spin echoes for a plurality views for each 90xc2x0 excitation.
A pulse sequence of the FSE technique is shown in FIG. 1. In FIG. 1, (1) is a sequence of 90xc2x0 and 180xc2x0 excitations; (2), (3) and (4) are sequences of slice gradients Gs, phase encoding gradients Gp and readout gradients Gr, respectively; and (5) is a sequence of spin echoes SE. These sequences proceed along a time axis t.
As shown, a 90xc2x0 excitation is effected while applying a slice gradient Gs1. Next, after a time period U1, a first 180xc2x0 excitation is effected while applying a slice gradient Gs2. Next, after a time period U2, a second 180xc2x0 excitation is effected while applying a slice gradient Gs3. Thereafter, third, fourth, . . . 180xc2x0 excitations are effected while applying respective slice gradients Gs4, Gs5, . . . at every time period U2 in a similar manner.
During the time period from the 90xc2x0 excitation to the first 180xc2x0 excitation, a readout gradient Gr1 is applied to perform phase dispersion, or dephasing, of the spins. Next, during the time period from the first 180xc2x0 excitation to the second 180xc2x0 excitation, a readout gradient Gr2 is applied to perform phase focusing, or rephasing, of the spins and generate a first spin echo SE1. The readout gradient Gr2, which generated the spin echo, dephases the spins in its latter half portion.
Prior to the application of the readout gradient Gr2, a phase encoding gradient Gp1 is applied to perform phase encoding, and after the application of the readout gradient Gr2 is completed, a phase encoding gradient Gp1xe2x80x2 is applied in the opposite direction to cancel the phase encoding.
Thereafter, readout gradients Gr3, Gr4, . . . are applied during every time period between the 180xc2x0 excitations to generate respective spin echoes SE2, SE3, . . . in a similar manner. Moreover, phase encoding is achieved by phase encoding gradients Gp2, Gp3 . . . The phase encoding is differentiated every time.
The spin echo is an RF signal having maximum amplitude at the center of the echo. The maximum amplitude, or a peak, of the first spin echo SE1 occurs after a time period TE (echo time) from the 90xc2x0 excitation. A peak of the second spin echo SE2 occurs after the time period TE from the peak of the first spin echo SE1. Thereafter, peaks of the spin echoes SE3, SE4, . . . occur at intervals of time period TE in a similar manner. The generation of a peak is sometimes referred to as focalization (image formation) of a spin echo.
In a magnet system that achieves static magnetic field generation by permanent magnets, residual magnetization may occur owing to, for example, magnetization of pole pieces of the permanent magnets by the gradient magnetic fields. Since the residual magnetization in the frequency axis direction affects the dephasing of the spins during the time period between the 90xc2x0 and 180xc2x0 excitations, timing of the spin echo focalization, or timing of the peak generation, experiences an error. A similar phenomenon is produced by eddy currents.
For example, if the first spin echo SE1 focalizes in a time period TExe2x80x2 shorter than proper TE, the second spin echo SE2 focalizes in a time period TExe2x80x3 longer than proper TE, and the third spin echo SE3 focalizes at timing TExe2x80x2 shorter than proper TE, due to the timing error in the focalization. Thereafter, the spin echo SEi (i: 4, 5, 6 . . . ) focalizes at timing such that a time period longer than proper TE and a time period shorter than proper TE alternate.
Since such a focalization error is a source of artifact generation in a reconstructed image, an attempt has been made to cancel the effect of the resident magnetization, eddy current etc. in the frequency axis direction by adjusting the readout gradient to correct the focalization error.
When residual magnetization exists also in the phase axis direction, a focalization error occurs due to the effect of the residual magnetization. The focalization error, however, cannot be corrected by adjusting the readout gradient because the axis of the gradient is different. Moreover, in the first place, the effect of the residual magnetization etc. in the phase axis direction on the phase of spins cannot be accurately measured.
It is therefore an object of the present invention to provide method and apparatus for accurately measuring a phase error in spins in the phase axis direction, method and apparatus for correcting such a phase error, a recording medium recorded with a program for operating a computer to perform a phase error measuring function, a recording medium recorded with a program to operate a computer to perform a phase error correcting function, a magnetic resonance imaging apparatus comprising phase error measuring means, and a magnetic resonance imaging apparatus comprising phase error correcting means.
(1) The present invention, in accordance with one aspect for solving the aforementioned problem, is a phase error measuring method characterized in comprising: effecting a 90xc2x0 excitation on object spins; effecting a first 180xc2x0 excitation after a first time period from said 90xc2x0 excitation; effecting a second 180xc2x0 excitation after a second time period from said first 180xc2x0 excitation; effecting a third 180xc2x0 excitation after said second time period from said second 180xc2x0 excitation; applying a gradient magnetic field having a time integral value of zero in a phase axis direction during the time period from said first 180xc2x0 excitation to said second 180xc2x0 excitation to read out a first spin echo signal; applying a gradient magnetic field having a time integral value of zero in the phase axis direction during the time period from said second 180xc2x0 excitation to said third 180xc2x0 excitation to read out a second spin echo signal; and determining a phase error of the spins in the phase axis direction during the time period from said 90xc2x0 excitation to said first 180xc2x0 excitation based on said first and second spin echo signals.
(2) The present invention, in accordance with another aspect for solving the aforementioned problem, is a phase error measuring apparatus characterized in comprising: spin exciting means for effecting a 90xc2x0 excitation on object spins, effecting a first 180xc2x0 excitation after a first time period from said 90xc2x0 excitation, effecting a second 180xc2x0 excitation after a second time period from said first 180xc2x0 excitation, and effecting a third 180xc2x0 excitation after said second time period from said second 180xc2x0 excitation; echo readout means for applying a gradient magnetic field having a time integral value of zero in a phase axis direction during the time period from said first 180xc2x0 excitation to said second 180xc2x0 excitation to read out a first spin echo signal, and applying a gradient magnetic field having a time integral value of zero in the phase axis direction during the time period from said second 180xc2x0 excitation to said third 180xc2x0 excitation to read out a second spin echo signal; and phase error calculating means for determining a phase error of the spins in the phase axis direction during the time period from said 90xc2x0 excitation to said first 180xc2x0 excitation based on said first and second spin echo signals.
(3) The present invention, in accordance with still another aspect for solving the aforementioned problem, is a recording medium characterized in being recorded in a computer-readable manner with a program for causing a computer to execute the functions of: effecting a 90xc2x0 excitation on object spins; effecting a first 180xc2x0 excitation after a first time period from said 90xc2x0 excitation; effecting a second 180xc2x0 excitation after a second time period from said first 180xc2x0 excitation; effecting a third 180xc2x0 excitation after said second time period from said second 180xc2x0 excitation; applying a gradient magnetic field having a time integral value of zero in a phase axis direction during the time period from said first 180xc2x0 excitation to said second 180xc2x0 excitation to read out a first spin echo signal; applying a gradient magnetic field having a time integral value of zero in the phase axis direction during the time period from said second 180xc2x0 excitation to said third 180xc2x0 excitation to read out a second spin echo signal; and determining a phase error of the spins in the phase axis direction during the time period from said 90xc2x0 excitation to said first 180xc2x0 excitation based on said first and second spin echo signals.
(4) The present invention, in accordance with still another aspect for solving the aforementioned problem, is a magnetic resonance imaging apparatus for acquiring a magnetic resonance signal from an object using a static magnetic field, gradient magnetic fields and a high frequency magnetic field, and constructing an image based on said acquired magnetic resonance signal, characterized in comprising: spin exciting means for effecting a 90xc2x0 excitation on object spins, effecting a first 180xc2x0 excitation after a first time period from said 90xc2x0 excitation, effecting a second 180xc2x0 excitation after a second time period from said first 180xc2x0 excitation, and effecting a third 180xc2x0 excitation after said second time period from said second 180xc2x0 excitation; echo readout means for applying a gradient magnetic field having a time integral value of zero in a phase axis direction during the time period from said first 180xc2x0 excitation to said second 180xc2x0 excitation to read out a first spin echo signal, and applying a gradient magnetic field having a time integral value of zero in the phase axis direction during the time period from said second 180xc2x0 excitation to said third 180xc2x0 excitation to read out a second spin echo signal; and phase error calculating means for determining a phase error of the spins in the phase axis direction during the time period from said 90xc2x0 excitation to said first 180xc2x0 excitation based on said first and second spin echo signals.
According to the invention in the aspects as described regarding (1)-(4), since no gradient magnetic field is applied in the phase axis direction during the time period between 90xc2x0 and 180xc2x0 excitations, and gradient magnetic fields having respective time integral values of zero are applied in the phase axis direction during the periods between 180xc2x0 excitations to read out respective spin echo signals, two spin echo signals can be obtained which are affected inversely with respect to each other by residual magnetization etc. in the phase axis direction during the time period between the 90xc2x0 and 180xc2x0 excitations. Thus, a phase error of spins can be determined from these spin echo signals via a calculation.
In the invention in the aspects as described regarding (1)-(4), by applying crusher gradient magnetic fields in the frequency axis direction before and after each 180xc2x0 excitation, interference by an FID signal can be eliminated and generation of a stimulated echo can also be inhibited to improve the accuracy of measurement.
(5) The present invention, in accordance with still another aspect for solving the aforementioned problem, is a phase error measuring method characterized in comprising: effecting a 90xc2x0 excitation on object spins; effecting a first 180xc2x0 excitation after a first time period from said 90xc2x0 excitation; effecting a second 180xc2x0 excitation after a second time period from said first 180xc2x0 excitation; effecting a third 180xc2x0 excitation after said second time period from said second 180xc2x0 excitation; applying a gradient magnetic field having a time integral value of zero in a phase axis direction during the time period from said first 180xc2x0 excitation to said second 180xc2x0 excitation to read out a first spin echo signal; applying a gradient magnetic field having a integral value of zero in the phase axis direction during the time period from said second 180xc2x0 excitation to said third 180xc2x0 excitation to read out a second spin echo signal; determining a phase error of the spins in the phase axis direction during the time period from said 90xc2x0 excitation to said first 180xc2x0 excitation based on said first and second spin echo signals; repeating said steps twice on trial with the polarity of said gradient magnetic fields inverted; and determining the average of the former and latter determined phase errors.
(6) The present invention, in accordance with still another aspect for solving the aforementioned problem, is a phase error measuring apparatus characterized in comprising: spin exciting means for effecting a 90xc2x0 excitation on object spins, effecting a first 180xc2x0 excitation after a first time period from said 90xc2x0 excitation, effecting a second 180xc2x0 excitation after a second time period from said first 180xc2x0 excitation, and effecting a third 180xc2x0 excitation after said second time period from said second 180xc2x0 excitation; echo readout means for applying a gradient magnetic field having a time integral value of zero in a phase axis direction during the time period from said first 180xc2x0 excitation to said second 180xc2x0 excitation to read out a first spin echo signal, and applying a gradient magnetic field having a time integral value of zero in the phase axis direction during the time period from said second 180xc2x0 excitation to said third 180xc2x0 excitation to read out a second spin echo signal; phase error calculating means for determining a phase error of the spins in the phase axis direction during the time period from said 90xc2x0 excitation to said first 180xc2x0 excitation based on said first and second spin echo signals; control means for repeating the spin excitations by said spin exciting means, the spin echo readout by said echo readout means, and the phase error calculation by said phase error calculating means twice on trial with the polarity of said gradient magnetic fields inverted; and average calculating means for determining the average of the former and latter determined phase errors.
(7) The present invention, in accordance with still another aspect for solving the aforementioned problem, is a recording medium characterized in being recorded in a computer-readable manner with a program for causing a computer to execute the functions of: effecting a 90xc2x0 excitation on object spins; effecting a first 180xc2x0 excitation after a first time period from said 90xc2x0 excitation; effecting a second 180xc2x0 excitation after a second time period from said first 180xc2x0 excitation; effecting a third 180xc2x0 excitation after said second time period from said second 180xc2x0 excitation; applying a gradient magnetic field having a time integral value of zero in a phase axis direction during the time period from said first 180xc2x0 excitation to said second 180xc2x0 excitation to read out a first spin echo signal; applying a gradient magnetic field having a time integral value of zero in the phase axis direction during the time period from said second 180xc2x0 excitation to said third 180xc2x0 excitation to read out a second spin echo signal; determining a phase error of the spins in the phase axis direction during the time period from said 90xc2x0 excitation to said first 180xc2x0 excitation based on said first and second spin echo signals; repeating said steps twice on trial with the polarity of said gradient magnetic fields inverted; and determining the average of the former and latter determined phase errors.
(8) The present invention, in accordance with still another aspect for solving the aforementioned problem, is a magnetic resonance imaging apparatus for acquiring a magnetic resonance signal from an object using a static magnetic field, gradient magnetic fields and a high frequency magnetic field, and constructing an image based on said acquired magnetic resonance signal, characterized in comprising: spin exciting means for effecting a 90xc2x0 excitation on object spins, effecting a first 180xc2x0 excitation after a first time period from said 90xc2x0 excitation, effecting a second 180xc2x0 excitation after a second time period from said first 180xc2x0 excitation, and effecting a third 180xc2x0 excitation after said second time period from said second 180xc2x0 excitation; echo readout means for applying a gradient magnetic field having a time integral value of zero in a phase axis direction during the time period from said first 180xc2x0 excitation to said second 180xc2x0 excitation to read out a first spin echo signal, and applying a gradient magnetic field having a time integral value of zero in the phase axis direction during the time period from said second 180xc2x0 excitation to said third 180xc2x0 excitation to read out a second spin echo signal; phase error calculating means for determining a phase error of the spins in the phase axis direction during the time period from said 90xc2x0 excitation to said first 180xc2x0 excitation based on said first and second spin echo signals; control means for repeating the spin excitations by said spin exciting means, the spin echo readout by said echo readout means, and the phase error calculation by said phase error calculating means twice on trial with the polarity of said gradient magnetic fields inverted; and average calculating means for determining the average of the former and latter determined phase errors.
According to the invention in the aspects as described regarding (5)-(8), since no gradient magnetic field is applied in the phase axis direction during the time period between 90xc2x0 and 180xc2x0 excitations, and gradient magnetic fields having respective time integral values of zero are applied in the phase axis direction during the periods between 180xc2x0 excitations to read out respective spin echo signals, two spin echo signals can be obtained which are affected inversely with respect to each other by residual magnetization etc. in the phase axis direction during the time period between the 90xc2x0 and 180xc2x0 excitations. Thus, a phase error of spins can be determined from these spin echo signals via a calculation.
By repeating such a phase error measurement twice on trial with the polarity of the gradient magnetic fields inverted, and averaging the resulting measured values, the effect of the gradient magnetic field applied during each time period between 180xc2x0 excitations on the measured value can be canceled, and measured values can be obtained with high accuracy.
In the invention in the aspects as described regarding (5)-(8), by applying crusher gradient magnetic fields in the frequency axis direction before and after each 180xc2x0 excitation, interference by an FID signal can be eliminated and generation of a stimulated echo can also be inhibited to further improve the accuracy of measurement.
In this case, by applying the crusher gradient magnetic fields with their polarity inverted in the two trials, the effect of the gradient magnetic fields applied as the crusher on the measured value can be canceled, thereby allowing measurement to be performed with higher accuracy.
(9) The present invention, in accordance with still another aspect for solving the aforementioned problem, is a phase error correcting method characterized in comprising: effecting a 90xc2x0 excitation on object spins; effecting a first 180xc2x0 excitation after a first time period from said 90xc2x0 excitation; effecting a second 180xc2x0 excitation after a second time period from said first 180xc2x0 excitation; effecting a third 180xc2x0 excitation after said second time period from said second 180xc2x0 excitation; applying a gradient magnetic field having a time integral value of zero in a phase axis direction during the time period from said first 180xc2x0 excitation to said second 180xc2x0 excitation to read out a first spin echo signal; applying a gradient magnetic field having a time integral value of zero in the phase axis direction during the time period from said second 180xc2x0 excitation to said third 180xc2x0 excitation to read out a second spin echo signal; determining a phase error of the spins in the phase axis direction during the time period from said 90xc2x0 excitation to said first 180xc2x0 excitation based on said first and second spin echo signals; and applying a gradient magnetic field that makes said phase error zero in the phase axis direction during a time period from a 90xc2x0 excitation to a 180xc2x0 excitation.
(10) The present invention, in accordance with still another aspect for solving the aforementioned problem, is a phase error correcting apparatus characterized in comprising: spin exciting means for effecting a 90xc2x0 excitation on object spins, effecting a first 180xc2x0 excitation after a first time period from said 90xc2x0 excitation, effecting a second 180xc2x0 excitation after a second time period from said first 180xc2x0 excitation, and effecting a third 180xc2x0 excitation after said second time period from said second 180xc2x0 excitation; echo readout means for applying a gradient magnetic field having a time integral value of zero in a phase axis direction during the time period from said first 180xc2x0 excitation to said second 180xc2x0 excitation to read out a first spin echo signal, and applying a gradient magnetic field having a time integral value of zero in the phase axis direction during the time period from said second 180xc2x0 excitation to said third 180xc2x0 excitation to read out a second spin echo signal; phase error calculating means for determining a phase error of the spins in the phase axis direction during the time period from said 90xc2x0 excitation to said first 180xc2x0 excitation based on said first and second spin echo signals; and corrective gradient magnetic field applying means for applying a gradient magnetic field that makes said phase error zero in the phase axis direction during a time period from a 90xc2x0 excitation to a 180xc2x0 excitation.
(11) The present invention, in accordance with still another aspect for solving the aforementioned problem, is a recording medium characterized in being recorded in a computer-readable manner with a program for causing a computer to execute the functions of: effecting a 90xc2x0 excitation on object spins; effecting a first 180xc2x0 excitation after a first time period from said 90xc2x0 excitation; effecting a second 180xc2x0 excitation after a second time period from said first 180xc2x0 excitation; effecting a third 180xc2x0 excitation after said second time period from said second 180xc2x0 excitation; applying a gradient magnetic field having a time integral value of zero in a phase axis direction during the time period from said first 180xc2x0 excitation to said second 180xc2x0 excitation to read out a first spin echo signal; applying a gradient magnetic field having a time integral value of zero in the phase axis direction during the time period from said second 180xc2x0 excitation to said third 180xc2x0 excitation to read out a second spin echo signal; determining a phase error of the spins in the phase axis direction during the time period from said 90xc2x0 excitation to said first 180xc2x0 excitation based on said first and second spin echo signals; and applying a gradient magnetic field that makes said phase error zero in the phase axis direction during a time period from a 90xc2x0 excitation to a 180xc2x0 excitation.
(12) The present invention, in accordance with still another aspect for solving the aforementioned problem, is a magnetic resonance imaging apparatus for acquiring a magnetic resonance signal from an object using a static magnetic field, gradient magnetic fields and a high frequency magnetic field, and constructing an image based on said acquired magnetic resonance signal, characterized in comprising: spin exciting means for effecting a 90xc2x0 excitation on object spins, effecting a first 180xc2x0 excitation after a first time period from said 90xc2x0 excitation, effecting a second 180xc2x0 excitation after a second time period from said first 180xc2x0 excitation, and effecting a third 180xc2x0 excitation after said second time period from said second 180xc2x0 excitation; echo readout means for applying a gradient magnetic field having a time integral value of zero in a phase axis direction during the time period from said first 180xc2x0 excitation to said second 180xc2x0 excitation to read out a first spin echo signal, and applying a gradient magnetic field having a time integral value of zero in the phase axis direction during the time period from said second 180xc2x0 excitation to said third 180xc2x0 excitation to read out a second spin echo signal; phase error calculating means for determining a phase error of the spins in the phase axis direction during the time period from said 90xc2x0 excitation to said first 180xc2x0 excitation based on said first and second spin echo signals; and corrective gradient magnetic field applying means for applying a gradient magnetic field that makes said phase error zero in the phase axis direction during a time period from a 90xc2x0 excitation to a 180xc2x0 excitation.
According to the invention in the aspects as described regarding (9)-(12), since a phase error is measured similarly to the invention in the aspects of (1)-(4), and a gradient magnetic field that makes the phase error zero is applied in the phase axis direction during a time period from a 90xc2x0 excitation to a first 180xc2x0 excitation, a phase error due to a residual gradient magnetic field in the phase axis direction can be corrected.
(13) The present invention, in accordance with still another aspect for solving the aforementioned problem, is a phase error correcting method characterized in comprising: effecting a 90xc2x0 excitation on object spins; effecting a first 180xc2x0 excitation after a first time period from said 90xc2x0 excitation; effecting a second 180xc2x0 excitation after a second time period from said first 180xc2x0 excitation; effecting a third 180xc2x0 excitation after said second time period from said second 180xc2x0 excitation; applying a gradient magnetic field having a time integral value of zero in a phase axis direction during the time period from said first 180xc2x0 excitation to said second 180xc2x0 excitation to read out a first spin echo signal; applying a gradient magnetic field having a time integral value of zero in the phase axis direction during the time period from said second 180xc2x0 excitation to said third 180xc2x0 excitation to read out a second spin echo signal; determining a phase error of the spins in the phase axis direction during the time period from said 90xc2x0 excitation to said first 180xc2x0 excitation based on said first and second spin echo signals; repeating said steps twice on trial with the polarity of said gradient magnetic fields inverted; determining the average of the former and latter determined phase errors; and applying a gradient magnetic field that makes the average of said phase errors zero in the phase axis direction during a time period from a 90xc2x0 excitation to a 180xc2x0 excitation.
(14) The present invention, in accordance with still another aspect for solving the aforementioned problem, is a phase error correcting apparatus characterized in comprising: spin exciting means for effecting a 90xc2x0 excitation on object spins, effecting a first 180xc2x0 excitation after a first time period from said 90xc2x0 excitation, effecting a second 180xc2x0 excitation after a second time period from said first 180xc2x0 excitation, and effecting a third 180xc2x0 excitation after said second time period from said second 180xc2x0 excitation; echo readout means for applying a gradient magnetic field having a time integral value of zero in a phase axis direction during the time period from said first 180xc2x0 excitation to said second 180xc2x0 excitation to read out a first spin echo signal, and applying a gradient magnetic field having a time integral value of zero in the phase axis direction during the time period from said second 180xc2x0 excitation to said third 180xc2x0 excitation to read out a second spin echo signal; phase error calculating means for determining a phase error of the spins in the phase axis direction during the time period from said 90xc2x0 excitation to said first 180xc2x0 excitation based on said first and second spin echo signals; control means for repeating the spin excitations by said spin exciting means, the spin echo readout by said echo readout means, and the phase error calculation by said phase error calculating means twice on trial with the polarity of said gradient magnetic fields inverted; average calculating means for determining the average of the former and latter determined phase errors; and corrective gradient magnetic field applying means for applying a gradient magnetic field that makes the average of said phase errors zero in the phase axis direction during a time period from a 90xc2x0 excitation to a 180xc2x0 excitation.
(15) The present invention, in accordance with still another aspect for solving the aforementioned problem, is a recording medium characterized in being recorded in a computer-readable manner with a program for causing a computer to execute the functions of: effecting a 90xc2x0 excitation on object spins; effecting a first 180xc2x0 excitation after a first time period from said 90xc2x0 excitation; effecting a second 180xc2x0 excitation after a second time period from said first 180xc2x0 excitation; effecting a third 180xc2x0 excitation after said second time period from said second 180xc2x0 excitation; applying a gradient magnetic field having a time integral value of zero in a phase axis direction during the time period from said first 180xc2x0 excitation to said second 180xc2x0 excitation to read out a first spin echo signal; applying a gradient magnetic field having a time integral value of zero in the phase axis direction during the time period from said second 180xc2x0 excitation to said third 180xc2x0 excitation to read out a second spin echo signal; determining a phase error of the spins in the phase axis direction during the time period from said 90xc2x0 excitation to said first 180xc2x0 excitation based on said first and second spin echo signals; repeating said steps twice on trial with the polarity of said gradient magnetic fields inverted; determining the average of the former and latter determined phase errors; and applying a gradient magnetic field that makes the average of said phase errors zero in the phase axis direction during a time period from a 90xc2x0 excitation to a 180xc2x0 excitation.
(16) The present invention, in accordance with still another aspect for solving the aforementioned problem, is a magnetic resonance imaging apparatus for acquiring a magnetic resonance signal from an object using a static magnetic field, gradient magnetic fields and a high frequency magnetic field, and constructing an image based on said acquired magnetic resonance signal, characterized in comprising: spin exciting means for effecting a 90xc2x0 excitation on object spins, effecting a first 180xc2x0 excitation after a first time period from said 90xc2x0 excitation, effecting a second 180xc2x0 excitation after a second time period from said first 180xc2x0 excitation, and effecting a third 180xc2x0 excitation after said second time period from said second 180xc2x0 excitation; echo readout means for applying a gradient magnetic field having a time integral value of zero in a phase axis direction during the time period from said first 180xc2x0 excitation to said second 180xc2x0 excitation to read out a first spin echo signal, and applying a gradient magnetic field having a time integral value of zero in the phase axis direction during the time period from said second 180xc2x0 excitation to said third 180xc2x0 excitation to read out a second spin echo signal; phase error calculating means for determining a phase error of the spins in the phase axis direction during the time period from said 90xc2x0 excitation to said first 180xc2x0 excitation based on said first and second spin echo signals; control means for repeating the spin excitations by said spin exciting means, the spin echo readout by said echo readout means, and the phase error calculation by said phase error calculating means twice on trial with the polarity of said gradient magnetic fields inverted; average calculating means for determining the average of the former and latter determined phase errors; and corrective gradient magnetic field applying means for applying a gradient magnetic field that makes the average of said phase errors zero in the phase axis direction during a time period from a 90xc2x0 excitation to a 180xc2x0 excitation.
According to the invention in the aspects as described regarding (13)-(16), since a phase error is measured similarly to the invention in the aspects of (5)-(8), and a gradient magnetic field that makes the phase error zero is applied in the phase axis direction during a time period from a 90xc2x0 excitation to a first 180xc2x0 excitation, a phase error due to a residual gradient magnetic field in the phase axis direction can be more accurately corrected.
Therefore, the present invention can provide a method and apparatus for accurately measuring a phase error in spins in a phase axis direction, a method and apparatus for correcting such a phase error, a recording medium recorded with a program for operating a computer to perform a phase error measuring function, a recording medium recorded with a program for operating a computer to perform a phase error correcting function, a magnetic resonance imaging apparatus comprising phase error measuring means, and a magnetic resonance imaging apparatus comprising phase error correcting means.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.